The Eleusinian Mysteries

Having recently read the Homeric Hymn to Demetre for only the second time, I am struck by the mystery at the very end

"She went then to the kings, keepers of custom,... And taught all of these her proper rites and mysteries-- Sacred, inviolable, not to be questioned or told of. Fear of the gods holds back the voice. Yet an earthly man is blessed who sees them. The uninitiated have no portion like his, when dead, below in mist and darkness."

I am struck by chills every time I read this passage. What rites and rituals did Demetre teach these people? For thousands upon thousands of years they have been at their mysterious work, yet all we outsiders can do is speculate as to anything that might be going on.

So then we must ask ourselves another question? How did they do it? How did they manage to keep the entirity of their practices quiet throughout the millenia, especially in today's world that is so much smaller than the world when the mysteries were first done? What respect or knowledge could help keep perhaps the greatest secret the world has ever known?